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<span id="Thread-safety"></span><div class="header">
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<hr>
<span id="Thread-safety-1"></span><h3 class="section">5.4 Thread safety</h3>

<span id="index-threads-1"></span>
<span id="index-OpenMP-3"></span>
<span id="index-thread-safety-1"></span>
<p>Users writing multi-threaded programs (including OpenMP) must concern
themselves with the <em>thread safety</em> of the libraries they
use&mdash;that is, whether it is safe to call routines in parallel from
multiple threads.  FFTW can be used in such an environment, but some
care must be taken because the planner routines share data
(e.g. wisdom and trigonometric tables) between calls and plans.
</p>
<p>The upshot is that the only thread-safe routine in FFTW is
<code>fftw_execute</code> (and the new-array variants thereof).  All other routines
(e.g. the planner) should only be called from one thread at a time.  So,
for example, you can wrap a semaphore lock around any calls to the
planner; even more simply, you can just create all of your plans from
one thread.  We do not think this should be an important restriction
(FFTW is designed for the situation where the only performance-sensitive
code is the actual execution of the transform), and the benefits of
shared data between plans are great.
</p>
<p>Note also that, since the plan is not modified by <code>fftw_execute</code>,
it is safe to execute the <em>same plan</em> in parallel by multiple
threads.  However, since a given plan operates by default on a fixed
array, you need to use one of the new-array execute functions (see <a href="New_002darray-Execute-Functions.html">New-array Execute Functions</a>) so that different threads compute the transform of different data.
</p>
<p>(Users should note that these comments only apply to programs using
shared-memory threads or OpenMP.  Parallelism using MPI or forked processes
involves a separate address-space and global variables for each process,
and is not susceptible to problems of this sort.)
</p>
<p>The FFTW planner is intended to be called from a single thread.  If you
really must call it from multiple threads, you are expected to grab
whatever lock makes sense for your application, with the understanding
that you may be holding that lock for a long time, which is undesirable.
</p>
<p>Neither strategy works, however, in the following situation.  The
&ldquo;application&rdquo; is structured as a set of &ldquo;plugins&rdquo; which are unaware
of each other, and for whatever reason the &ldquo;plugins&rdquo; cannot coordinate
on grabbing the lock.  (This is not a technical problem, but an
organizational one.  The &ldquo;plugins&rdquo; are written by independent agents,
and from the perspective of each plugin&rsquo;s author, each plugin is using
FFTW correctly from a single thread.)  To cope with this situation,
starting from FFTW-3.3.5, FFTW supports an API to make the planner
thread-safe:
</p>
<div class="example">
<pre class="example">void fftw_make_planner_thread_safe(void);
</pre></div>
<span id="index-fftw_005fmake_005fplanner_005fthread_005fsafe"></span>

<p>This call operates by brute force: It just installs a hook that wraps a
lock (chosen by us) around all planner calls.  So there is no magic and
you get the worst of all worlds.  The planner is still single-threaded,
but you cannot choose which lock to use.  The planner still holds the
lock for a long time, but you cannot impose a timeout on lock
acquisition.  As of FFTW-3.3.5 and FFTW-3.3.6, this call does not work
when using OpenMP as threading substrate.  (Suggestions on what to do
about this bug are welcome.)  <em>Do not use
<code>fftw_make_planner_thread_safe</code> unless there is no other choice,</em>
such as in the application/plugin situation.
</p><hr>
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